Cyrus, 17, told her audience that the new song, "Full Circle," is "about two people. They're always gonna come back together no matter what anyone says or the bad people that try to keep you apart."
"Surprise, surprise," she added, "It's about a Jonas brother . . . you didn't hear that."
Among the song's lyrics: "I try to run but I keep on coming back full circle and I can't jump the track, can't let you go."
In 2008, Cyrus told Seventeen magazine that she "bawled for a month straight" and dyed her hair black after breaking up with Jonas, also 17, at the end of 2007.
The couple reconnected last June after Cyrus broke up with model Justin Gaston.
At the time, she told Us Weekly that she and Jonas "definitely reconnected. We don't know what's going to happen in the future, but right now we're just kind of kicking it and hanging out as much as we can."
Cyrus is currently dating Liam Hemsworth, 20, her Australian co-star in "The Last Song."
Back in March, she gushed to the magazine about Hemsworth, saying "I think we're both deeper than normal people. What they think and how they feel."
But given her new song, perhaps Cyrus' feelings have since changed.